A mechanical halftone screen comprises a plurality of pinholes (there are several types of pinholes such as square type or chain type) arranged in matrix. A halftone dot generator of an electronic image reproducing system has a function of electronically generating halftone dots being equivalent to said mechanical halftone screen.
FIG. 1 shows an electronically-generated halftone dot. Precisely, one halftone dot corresponding to an area (W.times.L) is composed of a plurality of halftone dot elements .omega..sub.1 l.sub.1 to .omega..sub.20 l.sub.20 (each halftone dot element is called a "halftone sub-cell"). The halftone sub-cell is blackened when the density value of the corresponding section of an original is more than a fixed density value indicated as a number in each of the smallest squares of FIG. 1. Each of the circumferential halftone sub-cells is given a higher threshold density level, while each of the central halftone sub-cells is given a lower density threshold level.
By the way, FIG. 1 shows only the threshold levels of density of the halftone sub-cells situated in one quarter of the halftone dot because the density distribution is symmetrical about axes A, B and C in the other quarters.
FIG. 2 shows a graph of the density distribution of the quarter area shown in FIG. 1, in which the longitudinal axis represents the halftone dot percentage (%), the lateral axis represents the number of the column (.omega.) and the parameter represents the number of the row (l). As is obvious from FIGS. 1 and 2, only the central area of a halftone dot is blackened when the density of corresponding area of the original is comparatively lower, while a greater part of a halftone dot is blackened when the density of corresponding area of the original is comparatively higher. In other words, the density of a portion of an original can be expressed by the occupation rate of the blackened area to the whole area of a halftone dot.
Basically, the density value of an area of an image can be expressed by the corresponding voltage within a certain range (for example, for 0 V to 6 V). To realize that, an image reproducing system sets up an upper limit (called a "highlight level" hereinafter) and a lower limit (called a "shadow level" hereinafter); whereby a halftone sub-cell corresponding to an area of an image of which voltage level is equal to or higher than the highlight level is recorded in 0% halftone dot density (hereinafter, this state is identified as one wherein the halftone sub-cell is whitened), while a halftone sub-cell corresponding to an area of the image of which the voltage level is equal to or lower than the shadow level is recorded in 100% halftone dot density (hereinafter, this state is referred to as one wherein the halftone sub-cell is blackened).
In this case, when the density value of one division of the original, which is submitted to a color computation process, is used for recording each of one-sixteenth areas I, II, III . . . XVI corresponds to 55% halftone dot density, the halftone dot is actually recorded as shown as a hatched area in FIG. 1.
FIGS. 3(b), (c) and (d) show reproduction images of an original shown in FIG. 3(a) recorded according to the density distribution pattern of FIG. 1. In FIGS. 3(b), (c) and (d), the areas identified by I, II, III . . . XVI of each reproduction image correspond to the areas identified by the same signs in FIG. 1. These areas are recorded according to the density distribution pattern of FIG. 1 by using the density values obtained from areas i, ii, iii . . . vii of FIG. 3(a) respectively. It is now assumed that the original A of FIG. 3(a) is a binary density image (consisting of a white portion of 0% halftone dot density and a black (hatched) portion of 100% halftone dot density), and the average density values of the areas i, ii and iii correspond to halftone dot density values of 21%, 55% and 85% respectively. Because the area I is whitened under a condition in which the density of the corresponding area of the original is less than 56% halftone dot density, all the halftone sub-cells thereof are whitened by the signal from the area I having 21% halftone dot density value.
Because the area II includes halftone sub-cells which are to be blackened when the density of the corresponding area of the original has more than 20% halftone dot density, about half of the halftone sub-cells thereof are blackened by the signal of 55% halftone dot density. Similarly, because the area III has the same density distribution as that of the area II, all the halftone sub-cells are blackened by the signal of 85% halftone dot density. Consequently, the reproduction image of FIG. 3(b) is obtained. In this case, the areas I and II take shapes that are quite different from the corresponding areas i and ii of the original.
Then, assuming that the hatched area of the original A has 50% halftone dot density value, the areas i, ii and iii have halftone dot density values of 10.5%, 27.5% and 42.5% respectively. Consequently, the reproduction image of FIG. 3(c) is obtained, wherein areas I and II are different from the corresponding areas i and ii of the original.
Further assuming that the hatched area of the original A has 80% halftone dot density, and the lower density area has 25% halftone dot density value, the areas i, ii and iii have halftone dot density values of 36.55%, 55.25% and 71.75% respectively while each of areas v, vi and vii has halftone dot density of 25%. Consequently, the reproduction image of FIG. 3(d) is obtained, which image is different from the original.
To resolve the above problem, the pickup area (one division) of which density signal is to be submitted to a color computation process at a time should be smaller. However, the reduction of the pickup area results in prolongation of the color computation time.
Although the color computation time can be reduced by outputting the density signals from a plurality of pickup areas at a time, however, it requires as many color computation units as that of the pickup areas to embody the above method because each of the computation units is capable of processing the density signal of one pickup area at a time.
Japanese Kokai No. 55-146582 discloses the method as shown in FIG. 14. When an image of a division 103 composed of a plurality of sections (for example 4.times.4 sections) arranged in matrix is reproduced, at first several dot patterns 101 are prepared beforehand as shown in FIG. 14(b). The area of each of the dot patterns 101 corresponds to that of one division 103 shown in FIG. 14(a), while each of the dot patterns 101 corresponds to a pixel of a certain density range. Then a reproduction image is recorded as in the following way.
For example, when the density value of a section P.sub.22 of the division 103 is (5/16).times.100%, a dot pattern 5 corresponding to the density is selected. Then the signal of the corresponding square P'.sub.22 is output to record the corresponding halftone sub-cell P".sub.22 of a photosensitive material. Meanwhile, when a section P.sub.14 (having a density value of (7/16).times.100%) is to be recorded, the corresponding halftone sub-cell P".sub.14 is recorded by using the dot signal of the corresponding square P'.sub.14 of a dot pattern 7.
Therefore, in this method, a certain number of dot patterns must be prepared for density variation of the divisions of an original. In addition, the method has a drawback that even the density of a section is high enough, the corresponding halftone sub-cell is not blackened except for a case when the corresponding halftone dot pattern has a blackening dot at the corresponding place.
Japanese Kokai No. 51-114133 discloses a method as shown in FIG. 15. At first, a signal (A) of the density value of each section of an original (shown in FIG. 15(a)) is added to a signal (B) of corresponding halftone sub-cell reference value (shown in FIG. 15(b)) to obtain a signal (C) (shown in FIG. 15(c)). Then by comparing the signal of (C) with a signal (E) of a threshold value (in this case, (E)=19.5) a signal (D) is obtained, expressed by a combination of values "1" (when (C)&gt;(E)) and "0" (when (C)&lt;(E)). This signal is used for controlling a recording beam.
The above method aims to prevent a moire pattern from appearing onto a reproduction image, therefore, the object of the method is different from that of this invention.